Apple is offering a discounted battery replacement for all iPhone 6 and above this year. Customers who had the exchange made last year at the regular price will now receive a credit note from Apple.
Apple also treats its customers fairly retroactively. Anyone who paid for a new battery at Apple between January 1, 2017 and December 28, 2017 will now receive a credit of 50 dollars. By July 27, 2018 at the latest you will be informed by e-mail if you are one of these customers. Apple will inform you how you will be credited. Anyone who is overlooked in this notification campaign has the chance to contact Apple until the end of 2018 and claim the 50 bucks.
To everyone else we say: if you have an iPhone with a weak battery, then this year you should take the chance and replace it directly at Apple for the current discounted price.